With reference to FIG. 1, a gas turbine engine of known configuration is generally indicated at 10, having a principal and rotational axis 11. The engine 10 comprises, in axial flow series, an air intake 12, a propulsive fan 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, a low-pressure turbine 17 and an exhaust nozzle 18. A nacelle 20 generally surrounds the engine 10 and defines the intake 12.
The gas turbine engine 10 works in the conventional manner so that air entering the intake 12 is accelerated by the fan 13 to produce two air flows: a first air flow into the high-pressure compressor 14 and a second air flow which passes through a bypass duct 21 to provide propulsive thrust. The high-pressure compressor 14 compresses the air flow directed into it before delivering that air to the combustion equipment 15.
In the combustion equipment 15 the air flow is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high and low-pressure turbines 16, 17 before being exhausted through the nozzle 18 to provide additional propulsive thrust. The high 16 and low 17 pressure turbines drive respectively the high pressure compressor 14 and the fan 13, each by suitable interconnecting shaft.
As can be seen, stages of the compressor 14 are consolidated in axial alignment to form a single drum.
There are two presently known methods for joining the compressor stages to form a drum. In a first, discs are welded together using known inertia welding methods (also commonly known as spin welding or rotary friction welding). In a subsequent process, circumferential arrays of removable blades are then fastened to the welded discs. The process is relatively low cost, quick and useful in joining non-similar materials. Since there is no melting of the weld surfaces, the microstructure of the welded component is less affected than with some other known welding processes.
FIG. 2 illustrates a drum manufactured using such a process. As can be seen the finished drum comprises a plurality of discs 23, 24, 25, each carrying a circumferential array of buckets 23a, 24a, 25a. Axially extending circumferential rims of each disc 23, 24, 25 are welded together at welds W1, W2, W3. As is known, to perform an inertia weld, it is necessary for the work pieces to be forged together after or during friction is applied to the weld surfaces by rubbing the surfaces together. For this to be done, sacrificial material 23b, 24b, 25b is provided in the region of the circumferential rims which can be used to hold the components in the inertia welding equipment during the process. This sacrificial material also assists in balancing forces experienced by the components during welding to ensure a weld of satisfactory quality. After completion of the welding operation, the sacrificial material is removed in a post process machining operation. Such subsequent machining operations typically result in scrappage of the removed material.
As can be seen in FIGS. 3 and 4, during the inertia welding process opposing axial forces are applied to axially facing surfaces of the sacrificial materials 23b, 24b, 25b, 26b of the discs 23, 24, 25 and spline 26. Opposing forces F3 are applied to load balancing portions of sacrificial material 23b and 24b to provide weld W1, opposing forces F2 are applied to load balancing portions of sacrificial material 24b and 25b to provide weld W2 and opposing forces F1 are applied to load balancing portions of sacrificial material 25b and 26b to provide weld W3. As can be seen paired surfaces to which the forces are applied are axial facing and have a significant overlap in a radial direction adjacent the weld surfaces to facilitate good alignment of the weld. As can be seen the radial heights of the welds vary so as to allow access to apply the required paired axial forces F1, F2, F3. During the process, the disks can be clamped by radially outward protruding portions of the sacrificial materials 23b, 24b, 25b, 26b. 
FIG. 4 shows the inertia welded product of the forging of FIG. 3 after the sacrificial material has been removed to provide a drum having buckets 23a, 24a, 25a into which blades can be secured.
It is known that electron beam welding can be used to join thin pieces of material. Unlike in inertial welding, there are no significant frictional or forging forces to overcome. The electron beam can be accurately targeted to the closely positioned weld surfaces to create a weld pool and subsequent fusion of the materials. The process does not require sacrificial material for the attachment of welding equipment and to balance forces in the weld zone during the process. However, since the process involves local melting it can be deleterious to the microstructure of the weld material for some high temperature materials. Furthermore, the process is not well suited to joining dissimilar materials and is not suited to the welding of all high temperature materials.
In a second known method, a plurality of blisks is preformed and in a subsequent step the blisks are welded together using electron beam welding (EBW). FIGS. 5 and 6 illustrate a drum manufactured using such a process. As can be seen in FIG. 5, near net shape forging 31, 32, 33 are provided and in a first step are cut down to form the 31a, 32a, 33a. In a subsequent step, the blisks 31, 32, 33 are welded together at welds W4 and W5 as shown in FIG. 6. As can be seen there is little variation in the radial height of the welds. Since EBW is a fusion welding method, there is no requirement for the opposing axial forces F1, F2, F3 that are needed in an inertial welding process. It will be appreciated that the near net shape forging and finished blisks used in the known EBW process are not well suited to quality inertia friction welding.
There is a need for a cost efficient, widely applicable method for manufacturing a drum, more particularly which is suited to the manufacture of a blisk drum for use in a gas turbine engine.